1. Field of the Invention
The present invention relates to a hydrogen peroxide-containing bleach liquor achieving a potent bleaching effect without containing silicates, and a bleaching method using the bleach liquor. The bleach liquor and the bleaching method can be utilized mainly in scouring and bleaching steps in the textile industry. They are also applicable to bleaching steps in the field of industries, including a bleaching step in the linen industry and a pulp bleaching step in the paper industry.
2. Description of the Related Art
Scouring and bleaching with hydrogen peroxide have found increasing use in recent years because these technologies are cost-effective, give off no odors, and cause no environmental pollution.
To raise the speed of bleaching with hydrogen peroxide, supply of heat is well known to be an important factor. A commercially available aqueous solution of hydrogen peroxide, however, decomposes at temperatures in excess of 60.degree. C., losing active oxygen. Thus, a substance which enhances the heat stability of hydrogen peroxide and imparts a better bleach is added to the bleach liquor. Known examples of this substance are a system containing both sodium silicate and magnesium ions, aminopolycarboxylic acids, and protein derivatives.
The sodium silicate-Mg ions system contributes to the stabilization and activation of hydrogen peroxide at high temperatures, attaining an excellent bleaching effect. The resulting water-insoluble silicate, however, develops scale, which becomes the cause of a silicate-associated trouble such as a stain on the bleaching apparatus or deposits on an article to be bleached. For instance, the article to be bleached is stiffened by the deposition of the silicate scale, thus damaging the fibers and diminishing their physical properties. Discoloration may also occur. The finished product may be devaluated from these causes.
Aminopolycarboxylic acids or protein derivatives contribute to the stability of hydrogen peroxide at high temperatures, but impart a lower degree of bleach, because their activating effect is low.
The appropriate pH of a bleach liquor in the fiber industry is said to be 11. At a pH above 11, hydrogen peroxide self-decomposes markedly, lowering the degree of bleach, and the increased alkalinity decreases the strength of a cotton fabric. At a pH below 11, the activity of hydrogen peroxide decreases, and the degree of bleach lowers. Bleaching of cotton or synthetic fibers, therefore, use the appropriate pH of 11 at a bleaching temperature of 100.degree. C. or higher. Animal fibers such as wool or silk, on the other hand, may be damaged by an alkali or deteriorated under heat, so that they are bleached at pH 9 to 10 and at a temperature of 80.degree. to 90.degree. C. at the sacrifice of the bleaching power. Under this situation, the bleaching potential of hydrogen peroxide is not fulfilled.